1. Field of the Invention
The present invention generally relates to voltage detecting technology, in particular, to a voltage detecting circuit and a voltage detecting method at least capable of solving the problem of unstable detection caused by process difference.
2. Description of Related Art
A common integrated circuit uses a voltage source, for example, a VDD. However, during the operation of the circuit, the voltage source will be turned on and off frequently, so for some circuits, the voltage state of the voltage source must be detected accurately.
Conventional voltage detecting circuits have various designs. A complex circuit design uses a band-gap circuit. However, the detected voltage will have severe shift with the change of processes when the band-gap circuit is not used.
FIG. 1 is a schematic view of a conventional voltage detecting circuit without a band-gap circuit. Referring to FIG. 1, the voltage detecting circuit includes a current source 100 connected to a voltage source VDD for providing a current I1. The current I1 flows through a MOS transistor (M1) 102. The MOS transistor (M1) 102 serves as a resistor. The current I1 produces a voltage VA at a node A. Further, a PMOS transistor (M3) 104 and an NMOS transistor (M2) 106 form an inverter with two gates connected to the node A. Formula (1) represents the trigger voltage mechanism of the inverter:
                              V          SW                =                              VDD            -                          V                              th                ,                                  M                  ⁢                                                                          ⁢                  3                                                      +                                          V                                  th                  ,                                      M                    ⁢                                                                                  ⁢                    2                                                              ×                                                                    β                                          M                      ⁢                                                                                          ⁢                      2                                                                            β                                          M                      ⁢                                                                                          ⁢                      3                                                                                                                1            +                                                            β                                      M                    ⁢                                                                                  ⁢                    2                                                                    β                                      M                    ⁢                                                                                  ⁢                    3                                                                                                          (        1        )            
in which β are characteristic parameters of common MOS transistors.
FIG. 2 is a schematic view of a waveform of an operation voltage signal of the voltage detecting circuit in FIG. 1. Referring to FIG. 2 together, a voltage signal 108 is a voltage at an output end (OUT), a voltage signal 110 is a voltage VA at the node A, and a voltage signal 112 is the variance of the voltage source VDD. During the increase of the VDD, the early stage 114 does not need to be considered. At the next stage 116, when the voltage VA starts to increase and is greater than a trigger voltage 118 of the inverter formed by the transistors 104 and 106, the output voltage OUT will transit. The decrease of the voltage VDD has the same principle.
The conventional circuit has a disadvantage that the trigger voltage will vary with the critical voltage Vth and the characteristic parameter β of the semiconductor devices. The critical voltage Vth and the characteristic parameter β may be different due to different processes, thus causing variances in the detection results.
Therefore, the voltage detecting circuit still needs to be further improved.